The oil sands of the Fort McMurray region in Northern Alberta constitute a major reservoir of heavy oil or bitumen.
These oil sands comprise water-wetted sand grains having flecks of bitumen and fine clay particles disposed in the interstices between the grains.
At the present time, there are two large scale, surface mining operations recovering bitumen from the oil sands. One is owned by the assignees of the present application. Both operations involve first mining the oil sand and then using the hot water extraction process to extract the bitumen from the as-mined oil sand.
In order to appreciate the attributes of the present invention, it is necessary to have a general understanding of the nature of the hot water extraction process. This process is described in the technical literature but will now be briefly described in terms of the specific circuit used in the present assignee's plant. This prior art circuit is schematically illustrated in FIG. 1. More particularly:
The as-mined oil sand is first mixed with hot water (.about.90.degree. C.) and NaOH, by passing it through a horizontal rotating drum (referred to as a tumbler), to produce a slurry having a temperature of about 80.degree. C. Typically the oil sand is mixed in the tumbler with about 20 wt. % hot water and 0.02 wt. % NaOH (both based on the weight of the oil sand). The tumbler retention time is about 3 minutes. Steam is sparged into the slurry as it moves through the tumbler, to ensure the desired slurry exit temperature. In the course of being mixed and cascaded during passage through the tumbler, the following mechanisms are thought to occur: the NaOH generates surfactants in situ by reacting with constituents of the bitumen; the temperature and viscosity of the bitumen changes and it separates from the sand grains and is dispersed into the water phase of the slurry; fine air bubbles are entrained in the slurry; bitumen flecks coalesce into larger flecks; and some bitumen flecks and air bubbles of comparable size contact and the bitumen coats the air bubble to produce buoyant aerated bitumen. The term "conditioning" is applied in the art to describe the sum of these various happenings; PA1 The product slurry is screened, to reject oversize material, such as rocks, and is then diluted with additional hot water to increase the slurry water content to about 60 wt. %; PA1 The diluted slurry is introduced into a large thickener-like vessel having a conical bottom section and open-topped cylindrical upper section. This vessel is referred to as the primary separation vessel ("PSV"). The residence time of the slurry in the PSV is approximately 45 minutes. During this time, the sand sinks, is concentrated in the conical section and is pushed by rakes to a bottom outlet and removed as a tailings underflow. Some bitumen is lost with this underflow. Simultaneously, much of the still non aerated bitumen becomes aerated in the PSV as the non-aerated and aerated bitumen flecks and air bubbles continue to contact, in many cases rendering the aerated bitumen sufficiently buoyant to rise through the PSV fluid column. The rising aerated bitumen forms a froth layer on the upper surface of the PSV. This froth overflows the top rim of the PSV and is recovered and led away in a launder. In the mid-section of the PSV is a watery suspension referred to as "middlings", containing flecks of non-buoyant bitumen and dispersed fine solids. A dragstream of middlings is withdrawn from the PSV through an outlet in its mid-section wall. Although there is variance in the composition of the middlings, they typically contain 1-2 wt. % bitumen; PA1 To recover the residual bitumen contained in the PSV tailings and middlings, they are combined and processed in what is referred to as the "post-primary circuit". More particularly, the combined stream is fed to a deep cone vessel referred to as the tailings oil recovery vessel ("TORV"). This vessel and its method of operation is disclosed in U.S. Pat. No. 4,545,892. Briefly stated, the incoming feed to the vessel is deflected and spread out laterally and contacted from below by an upwelling stream of aerated and recycled TORV middlings. The air bubbles in the recycled middlings contact and aerate previously non-buoyant bitumen in the feed and a "secondary" bitumen froth is produced. This secondary froth is more contaminated with water and solids than primary froth from the PSV. The TORV secondary froth is recovered by overflowing the lip of the TORV and being led away in a launder. A solids-rich tailings stream, low in bitumen content, is produced as a TORV underflow. A middlings stream is withdrawn from the mid-section of the TORV. Part of this TORV middlings stream is aerated and recycled, as aforesaid. PA1 The balance of the TORV middlings is forwarded to a bank of impeller-agitated, sub-aerated flotation cells. In these mechanical flotation cells, the TORV middlings are relatively vigorously agitated and aerated to produce a heavily contaminated flotation froth, together with a tailings underflow; PA1 The flotation froth is temporarily retained in a tank, to allow some solids and water to settle out. The "cleaned" flotation and TORV froth are then combined with the PSV froth to yield a product stream that is subjected to two stages of centrifugation, to remove contained water and solids, thereby producing clean bitumen ready for refinery upgrading. PA1 That the air is supplied in the form of fine bubbles; PA1 That the air bubbles and bitumen flecks are of approximately equal size, perhaps having a diameter in the order of 1 mm; PA1 That the air be supplied in sufficient amount; and PA1 That the air bubbles and bitumen flecks have an opportunity to mix in a manner whereby contact between them is promoted. PA1 a venturi tube connected to the plenum at an inlet; PA1 a tubular nozzle member positioned close to, but gapped from the venturi tube inlet, said nozzle member being connected by a line with a pump for the supply of recycled TORV middlings, to create a liquid jet issuing from the nozzle member outlet; and PA1 a tubular sparger positioned in an outwardly spaced, concentric relation about the nozzle member outlet, said sparger being connected by a line with a source of pressurized air, for supplying a high velocity, annular stream of air surrounding the middlings jet; PA1 Maintenance costs can be significantly reduced because the eductor/aerator assemblies are external of the PSV and are readily accessible for servicing; PA1 Production losses can be reduced because only a single eductor/aerator assembly that requires service needs to be shut down--the remaining assemblies can remain in service; PA1 The eductor/aerator assemblies operate at optimum efficiency, since poor performing eductor/aerator assemblies can be detected and repaired easily; PA1 Capital and operating costs can be reduced by eliminating the post-primary circuit or minimizing its usage; and PA1 Overall water consumption can be reduced, due to recycling the middlings.
It will be appreciated from the foregoing description that the fundamental recovery mechanism in the hot water extraction process is air flotation. In order to achieve an adequate recovery using flotation, it is desirable:
Stated otherwise, a sufficient number of air bubbles and bitumen flecks must contact and unite to create a commercially viable yield of sufficiently buoyant bitumen which is able to rise through the vessel contents within the allocated retention time and be recovered as froth.
It is always unpredictable, when attempting a new approach to aeration in the context of the hot water extraction process, whether a sufficient number of the bitumen flecks will, in the first instance, contact bubbles of air and, in the second instance, whether a contacting bubble and fleck will unite to produce sufficiently buoyant aerated bitumen.
At this point, it is appropriate to point out that the applicant has used, in the present invention, some features of an aeration technique taught in U.S. Pat. No. 4,545,892 (The TORV patent). In that patent, it is taught to suspend a plenum or hollow vessel in a submerged state in the TORV chamber. Eductor/aerator devices are mounted to openings leading into the lower end of the plenum. These eductor/aerator devices function to inject air bubbles and recycled TORV middlings into the plenum. The produced aerated mixture exits the upper end of the plenum through outlets, to provide the previously mentioned upwelling stream. Each eductor/aerator device comprises:
whereby the motive jet of recycled TORV middlings induces a flow of unaerated middlings from the TORV chamber through the gap formed between the nozzle member and venturi tube, and the injected air flow is dispersed by the jet into the form of fine air bubbles that mix with the middlings in the plenum.
This TORV aeration system has worked adequately on a commercial basis. However it will be appreciated that, if the eductor/aerator devices become plugged or damaged, it is a major operation to make the necessary repairs. The TORV has to be shut down and drained to enable access to the normally submerged devices. This results not only in labour and repair costs but, more importantly, in lost production.
With this background in mind, it is now appropriate to describe the present invention.